Factorization studies in semi-inclusive DIS

1
Factorization studies in semi-inclusive DIS
H.Avakian (JLab)
Workshop on Parton Distributions in the 21st
Century November 19, 2004

• Semi-inclusive processes
• Factorization at low energies
• Kinematic dependences of A1
• PT dependence of spin and azimuthal asymmetries
• Transition from semi-inclusive to exclusive
  processes and HT effects
• Summary Outlook

) in collaboration with P.Bosted and
L.Elouadrhiri
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Semi-inclusive DIS at JLab
X.Ji

• study the transition between the nonperturbative
  and perturbative regimes of QCD
• test factorization and identify the limits of the
  partonic description
• measure PDFs and study higher twists

Main focus of SIDIS studies

• orbital angular momentum of quarks
• parton distributions at large x

3
SIDIS kinematic plane and relevant variables
Trento convention
4
SIDIS studies require

• Factorization in SIDIS into a distribution
  function f (x) fragmentation function D(z) and
  hard scattering part.
• Universality of distribution and fragmentation
  functions
• (SIDIS,DY,ee-)

• Applicability of partonic description at JLab
  energies
• Low W and Q2
• Low MX in ep-gtehX
• Control over background processes
• Contamination from target fragmentation
• Exclusive vector mesons

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SIDIS (gp-gtpX) x-section at leading twist
TMD PDFs
f1
h-1
g1
T-odd
h-1L
f-1T
g1T
h1
h-1T
kT-even
kT-odd
The structure functions depend on Q2, xB, z, PhT
1) Unpolarized target 2) Longitudinally pol.
target 3) Transversely pol. target
Studies of PDFs require three experiments
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SIDIS factorization
Structure functions FUU,FLL factorize into TMD
distribution and fragmentation
QCD Factorization (Ji and col.)
Naïve Factorization (Mulders and col.)
TMD FF
p
hard scattering
soft factor
k
TMD PDF
SIDIS Factorization proved for low PT !!!
P- has contributions from distribution,
fragmentation and soft gluon radiation
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The CLAS Detector
Scattering of 5.7 GeV polarized electrons off
polarized NH3 and unpolarized hydrogen (EG2000)

• 8M p in DIS kinematics,SIDIS
  Q2gt1.1GeV2,W2gt4,ylt0.85,
• beam polarization 73
• target polarization 72 (f0.2)

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Missing mass of pions in ep?epX
p0
p-
p
n
D
D0
MXgt1.5 (Q2gt1.1,W2gt4) may be applied to exclude
the kinematics where resonances in the remnant
may have significant contribution
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Multiplicities and Factorization
Phys.Rev.D69112004,2004
Dup KKP (LO)
No significant variation observed in z
distributions of p for different x ranges
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Data vs MC A1p
(filled circles sum over all x)
DIS cuts for ep-gt e p X Ee 5.7 GeV, Q2gt1.3,
W2gt4, MX2 gt 1.1, ylt0.85, 0.4ltzlt0.7
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A1p for p z-dependence
EG2000
PEPSI-MC
Data shows no significant z-dependence in the
0.4ltzlt0.7 range (consistent with PEPSI-MC)
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A1p for p- z-dependence
PEPSI-MC
EG2000
Double spin asymmetry decreases with z,
consistent with PEPSI-MC
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A1p for p0 z-dependence
EG2000
PEPSI-MC
Data consistent with PEPSI-MC
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A1p for p/-/0 z-dependence
PEPSI-MC
EG2000

• Data shows no significant Q2-dependence
  (consistent with LUND-MC)
• at large z p/- asymmetries lower than in LUND-MC
• at small and large z p0 looks higher than p

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A1p for p/-/0 z-dependence on MX-cut
EG2000

• No significant effect from MXgt1.5(filled symbols)
  cut compared to MXgt1.05(open symbols)
• MXgt1.5 cuts zgt0.8, doesnt affect the
  z-dependence of A1

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A1p for p x-dependence
In the z-range 0.4-0.7 (filled circles) no
observable effect
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A1p for p- x-dependence
At large z significant difference
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A1p for p0 x-dependence
In the z-range 0.4-0.7 no observable effect
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A1p data vs PEPSI-MC
ep ? e p X ( Ee 5.7 GeV, MX gt 1.1)
A1p
0.4ltzlt0.7

• x dependence of CLAS A1p (A20) consistent with
  HERMES data and partonic description.
• Measurements with different hadrons allow flavor
  separation

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A1p for p/-/0 PT-dependence
EG2000
A1p
No significant effect from MXgt1.5 cut compared to
MXgt1.05 in PT-dep.
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g1/f1 for p/-/0 PT-dependence
EG2000/PEPSI-MC
PRELIMINARY
PRELIMINARY
No dependence on Q2 (consistent with PEPSI-MC)
Asymmetries from kT-odd (f1-, h1-, gT..) and
kT-even (g1) distribution functions are expected
to have a very different behavior
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SSA PT-dependence of sinf moment
ssinfLU(UL) FLU(UL) 1/Q (Twist-3)
CLAS _at_4.3 GeV
CLAS _at_5.7 GeV
PRELIMINARY

• Beam and target SSA for p (not sensitive to
  MX-cut) are consistent with increase with PT
• first measurement of non-0 effect on p- for Lpol.
  proton.
• consistent with HERMES claim of significant
  unfavored H1 PFF

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pion SSA from r(pp-/pp0)
PYTHIA at 5.7 GeV
(CLAS _at_5.7GeV)
r0
Larger fraction of p from r at low x and large z
p SSA at large z may also have a significant
(20) contribution from r
Exclusive r (higher twist for SIDIS) crucial for
pX and ppX studies
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Transverse Target SSA at CLAS _at_11 GeV
CLAS _at_ 11GeV
Collins
AUT
Sivers
AUT
x4
Sivers effect (unpolarized quarks) is
kinematically enhanced.
Simultaneous (with pion SIDIS) measurement of,
exclusive r,r,w,g with a transversely polarized
target important to control the background.
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Summary

• Current CLAS data are consistent with a partonic
  picture
• The data consistent with factorization (no
  x/z-dependence observed in single and double spin
  asymmetry measurements).
• Spin and azimuthal asymmetries extracted for
  SIDIS p are consistent with theory predictions
  and measurements at higher energies.
• Superior statistics allows extraction of
  kinematic dependences of accessible spin and
  azimuthal asymmetries in SIDIS.

• Large acceptance and high luminosity of JLab will
  allow global analysis of SSA for exclusive and
  semi-inclusive final states measured in a wide
  range of PT,f,x,Q2 with separation of
  contributions from different mechanisms needed
  for precision measurements of underlying
  distribution functions.

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Questions to address in SIDIS

• How do we test the factorization and quantify its
  breakdown
• Kinematic limits of SIDIS (Q2gt1,W2gt4)
• The MX cut in ep?epX, and contribution from
  semi-exclusive events
• The contribution to pion SIDIS from exclusive
  vector mesons
• What we learn from PT-dependence of observables
• What we learn from target fragmentation (xFlt0)
  studies

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Target SSA sinf moment
At least 3 possible contributions to study

• First measurement of non-0 effect on p- for Lpol.
  proton.
• SSA consistent with HERMES claim of significant
  unfavored H1 PFF
• For flavor decomposition of TMD distributions
  need more data

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support slides..
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Longitudinally polarized Target SSA for p at
11GeV
Measurement of kT dependent twist-2 distribution
will check the existence of the Collins
fragmentation.

• Study the PT dependence of AULsin2f
• Study the possible effect of large unfavored
  Collins function.

Efremov et al.
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A1p data vs PEPSI-MC
ep ? e p X ( Ee 5.7 GeV, MX gt 1.1)

• No z-dependence of A1p observed above z0.4

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Transverse momentum of quarks

• kT crucial for orbital momentum and spin
  structure studies
• lead to SSA in hard scattering processes
• kT lead to 3 dimensional description
• lead to introduction of kT dependent/u PDFs
• kT - important for cross section description
  (important also for exclusive production)
• PT distributions of hadrons in DIS
• exclusive photon production (DVCS)
• - hard exclusive vector meson x-section
• - pp ? p0X (E704,RHIC) x-sections

To study orbital motion of quarks in
semi-inclusive DIS measurements in a wide range
of PT, f and fS are required.
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SIDIS kinematics
Unpolarized Long. pol Trans. pol
Kinematic pre-factors depend on the beam energy
yn/E
CLAS12
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ALU x-dependence CLAS _at_ 4.3 and 5.7 GeV
0.5ltzlt0.8
Beam SSA analyzed in terms of the Collins effect
by Schweitzer et al. using e(x) from cQSM
Separation of different HT contribution will
require independent measurements of separate
contributions
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Universality of Distribution Fragmentation
functions
J. Collins
kT-dependent distribution and fragmentation
functions are universal !
Brodsky et al. ? h-1 responsible for measured
cos2f in DY (CERN 1986-87 , Fermilab
1989)
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Target SSA from EG2000 sinf and sin2f
The sinf moment from data (unlike sin2f) is
bigger than Collins mechanism alone can
provide.

• Dashed lines for Collins mechanism
• Dotted line Sivers effect
• Solid line total

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Double spin asymmetries with transverse target
L.Gamberg et al. Phys.Lett.B584276-284,2004
access gT and its PT-dependence
6 GeV
negligible
12 GeV
27.5 GeV
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SIDIS
If Sivers dominates, the same SSA with - sign
should be in the target fragmentation region
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Exclusive pp- and pp0
e p e p ?
p p-
n
r
r0
r0 PID by Mpp and missing p-
r PID by Mpp and missing neutron
Fit Mpp with a BW polinomial background.
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Multiplicities and Factorization
LO,NLO NNLO KSV
Dup KKP (LO)
No significant variation observed in z
distributions of p for different x ranges
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Contributions to s in Polarized SIDIS
Twist-2 TMDs from Mulders et al
sUU
sLL
Sivers
sUT
sUT
Collins
sUL
KM
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g1/f1 for p PT-dependence
EG2000-PRELIMINARY
Data consistent with PEPSI-MC
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g1/f1 for p- PT-dependence
EG2000
Data consistent with PEPSI-MC
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g1/f1 for p0 PT-dependence
EG2000
Data consistent with PEPSI-MC
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Multiplicities and Factorization
LO,NLO NNLO KSV
Dup KKP (LO)
HERMES multiplicities
No significant variation observed in z
distributions of p for different x ranges
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Transverse Target SSA at CLAS _at_11 GeV
CLAS _at_ 11GeV
Collins
AUT
Sivers
AUT
x4
Sivers effect at CLAS (the main focus of SIDIS
studies) is also kinematically enhanced.
Simultaneous measurement of SIDIS, exclusive
r,r,w and DVCS asymmetries with a transversely
polarized target.
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g1/f1 for p/-/0 z-dependence
EG2000
PEPSI-MC
In the z-range 0.4-0.7 no observable effect
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g1/f1 for p/-/0 z-dependence
EG2000
PEPSI-MC
In the z-range 0.4-0.7 no observable effect
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g1/f1 for p/-/0 PT-dependence
EG2000
PEPSI-MC
Different x-bins may have different PT-dependence
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PT-dependence
MC
corrected for ltxgt
A1p for Egt5GeV doesnt show significant PT
dependence
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double spin asymmetry A1p
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NH3 dilution
Dilution function extracted from PEPSI-MC is
consistent with data
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g1/f1 for p/-/0 z-dependence
EG2000
In the z-range 0.4-0.7 no observable effect
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PT-dependence
A1p for Egt5GeV runs.
No significant pT dependence (filled circles sum
over all z)
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Quantum Phase-Space Distributions of Quarks
Probability to find a quark u in a nucleon P with
a certain polarization in a position r and
momentum k
Wpu(k,r) Mother Wigner distributions
d3r
d2kT dr
GPDs Hpu(x,x,t), GDAs Fudp(x,x)
FT
TMD PDFs fpu(x,kT), TMD FFs Dup(kT,z)
IPDs Wpu(x,rT),
GPD
x0
x0,t0
d2r
dx
d2kT
Measure momentum transfer to target Direct info
about spatial distributions
Measure momentum transfer to quark Direct info
about momentum distributions
PDFs fpu(x), FFs Dup(z)
Form Factors F1pu(t),F2pu(t )..
Some PDFs same in exclusive and semi-inclusive
analysis